1. Field of the Invention
This invention relates to an image forming apparatus such as a copying apparatus or a laser printer using, for example, the electrophotographic system or the electrostatic recording system, and a cartridge.
2. Related Background Art
In recent years, image forming apparatuses utilizing an electrophotographic system, such as copying apparatuses, laser beam printers and LED printers, have been widely used. Particularly with the recent spread of personal computers, inexpensive and compact image forming apparatuses have been desired as page printers for personal use.
At present, however, image forming apparatuses of this kind cannot be said to be sufficiently satisfactory in term of the quality of the produced image, and a higher quality of image is desired. As one means for satisfying such a requirement, there is desired an image forming apparatus improved in resolution, that is, provided with a high pixel density of 600 and 1200 dpi and further, 2400 dpi or greater.
On the other hand, in order to cope with diversified types of printing, the demand for an apparatus that can change over pixel density with a single image forming device to obtain an optimum image, depending on the purpose of printing, is heightening.
In such an apparatus, there is known a method of changing over the spot diameter of an exposure beam (the diameter for which the output of the exposure beam becomes 1/e.sup.2 of a maximum value) to thereby change over the pixel density.
However, when the spot diameter is made small in accordance with the need for high resolution, the proper spot diameter is, for example, about 40 .mu.m for 600 dpi, and about 20 .mu.m for 1200 dpi. To achieve such small spot diameters, a highly accurate aperture mechanism is required and further, as the spot diameter becomes smaller, the depth of focus of the exposure beam becomes shallower and therefore, a highly accurate optical system becomes necessary to form an image on a latent image bearing member, and a great increase in cost has become unavoidable.
So, as a method of changing over the pixel density without any increase in the cost of the optical system, there is a method of changing over the light amount of the exposure beam without changing the spot diameter. In the case of this method, the changeover of the light amount of the exposure beam can be done simply by changing the driving current of a light emitting element, and it is possible to change over the pixel density without increasing the cost in particular.
However, when the light amount is changed over by the use of a conventional photosensitive drum, if the light amount is adjusted so that the line width may become proper, there has arisen the problem that a small dot image, formed by one to several dots during high resolution, i.e., when the light amount set is small, is not reproduced.
FIG. 7 of the accompanying drawings is a graph showing the relations among the light amount distribution of the exposure beam, the sensitivity characteristic of the drum and an electrostatic latent image formed on the photosensitive drum. The first quadrant is the light amount distribution of the exposure beam, and the axis of the abscissas thereof represents the position, and the axis of the ordinates thereof represents the light amount. The second quadrant is the sensitivity characteristic of a popular photosensitive drum, and the axis of the ordinates thereof represents the light amount, and the axis of the abscissas represents the potential of the photosensitive drum. The third quadrant is the potential distribution of an electrostatic latent image projected with the sensitivity characteristic of the photosensitive drum added to the light-amount distribution of the exposure beam, and the axis of the abscissas thereof represents the potential and the axis of the ordinates represents the position.
A description will hereinafter be made of a case where the changeover of the resolution is set so as to change over, for example, at 600 dpi and 1200 dpi, and during 1200 dpi, the light amount of the exposure beam is set to about a half of that during 600 dpi.
The size of the electrostatic latent image to be developed must be substantially proportional to the size of a pixel determined by the resolution, and this condition can be satisfied by setting the developing bias to the potential of Vdev. In this setting, however, the developing contrast during 1200 dpi is very small and the latent image is hardly developed.
Also, when a half tone by lateral lines is printed by an image forming apparatus using a charging device for applying an AC bias to thereby charge the surface of a photosensitive drum, if the frequency of the bias is low to a certain extent, the period of the bias and the period of the lateral lines interfere with each other, whereby belt-like light and shade on an image (hereinafter referred to as the Moire) occur.
Accordingly, at a high pixel density, such as 1200 dpi, the frequency of the charging bias is preset to a high level and the moire can be made practically unseen, but if the frequency is made high, electrical damage to the surface of the drum will increase and the amount of scraping will become great.